Cold accumulating material for extremely low temperature cold, refrigerator using the same and heat shielding member

ABSTRACT

A cold accumulating material for extremely low temperature cold 1 is formed by filling pores 3 of a porous carrier 2 with magnetic particles 4 containing a rare earth element. The porous carrier 2 is desired to be formed of a sheet-shaped porous metal. A refrigerator according to this invention contains a cold reserving unit 5 filled with the aforementioned cold accumulating material for extremely low temperature cold 1. The above described structure is capable of exerting a sufficient refrigerating capacity with a small pressure loss of refrigerant (operating medium) and providing a cold accumulating material for extremely low temperature cold easy to process to a shape reducing the pressure loss and a refrigerator using the same.

FIELD OF THE INVENTION

The present invention relates to a cold accumulating material forextremely low temperature cold for use in refrigerators or the like, arefrigerator using the same and an extremely low temperature heatshielding member, and more particularly to a cold accumulating materialfor extremely low temperature cold in which a pressure loss ofrefrigerant is so small that a refrigerating performance thereof can beexerted sufficiently and which is easy to process to a configurationreducing the pressure loss, a refrigerator using the same and anextremely low temperature heat shielding member.

BACKGROUND ART

Recently, superconductivity technology has been progressed remarkablyand with an expanding application field thereof, development of a small,high performance refrigerator has become indispensable. For such arefrigerator, light weight, small size and high heat efficiency aredemanded.

For example in a superconductive MRI apparatus, cryopump and the like, arefrigerator based on such refrigerating cycle as Gifford MacMahon type(GM refrigerator), Starling method has been used. Further, a magneticfloating train absolutely needs a high performance refrigerator.Further, in recent years, a superconductive power storage apparatus(SMES) or a in-magnetic field single crystal pull-up apparatus has beenprovided with a high performance refrigerator as a main componentthereof. Further, to stabilize the temperature of a component materialoperating under ultra-low temperatures such as a superconductive wire,superconductive device, infrared ray sensor, the cold accumulatingmaterial for extremely low temperature cold for a thermal anchor, heatsink and heat shielding has been widely used.

FIG. 9 is a sectional view showing a main structure of a conventionaltwo-staged GM refrigerator. This GM refrigerator 10 has a vacuumcontainer 13 containing a first cylinder 11 having a large diameter anda second cylinder 12 connected coaxially to the first cylinder 11. Thefirst cylinder 11 contains a first cold accumulating unit 14 which isfreely reciprocatable and the second cylinder 12 also contains a secondcold accumulating unit 15 which is freely reciprocatable. Seal rings 16,17 are disposed between the first cylinder 11 and first coldaccumulating unit 14, and between the second cylinder 12 and second coldaccumulating unit 15 respectively.

The first cold accumulating unit 14 accommodates a first coldaccumulating material 18 made of Cu mesh or the like. The second coldaccumulating unit 15 contains a second cold accumulating material 19made of a cold accumulating material for extremely low temperature cold.The first cold accumulating unit 14 and second cold accumulating unit 15have operating medium (refrigerant) paths for He gas or the like whichare provided in gaps of the first cold accumulating material 18 and coldaccumulating material for extremely low temperature cold 19.

A first expansion chamber 20 is provided between the first coldaccumulating unit 14 and second cold accumulating unit 15. A secondexpansion chamber 21 is provided between the second cold accumulatingunit 15 and an end wall of the second cylinder 12. A first cooling stage22 is provided on a bottom of the first expansion chamber 20 and furthera second cooling stage 23 which is colder than the first cooling stage22 is provided on a bottom of the second expansion chamber 21.

A high pressure operating medium (e.g., He gas) is supplied from acompressor 24 to the aforementioned two-staged GM refrigerator 10. Thesupplied operating medium passes through the first cold accumulatingmaterial 18 accommodated in the first cold accumulating unit 14 andreaches the first expansion chamber 20, and further passes through thesecond cold accumulating material (second cold accumulating material) 19accommodated in the second cold accumulating unit 15 and reaches thesecond expansion chamber 21. At this time, the operating medium suppliesheat energy to the respective first cold accumulating materials 18, 19so that they are cooled. The operating medium passing through therespective first cold accumulating materials 18, 19 is expanded in therespective first expansion chambers 20, 21 so as to produce coolatmosphere thereby cooling the respective cooling stages 22, 23. Theexpanded operating medium flows in the respective cold accumulatingmaterials 18, 19 in opposite direction. The operating medium receivesheat energy from the respective cold accumulating materials 18, 19 andis discharged. As recuperation effect is improved in this process, theheat efficiency of the operating medium cycle is improved so that afurther lower temperature is realized.

That is, in the above described GM refrigerator, the operating mediumsuch as compressed He gas flows in a single direction in a coldaccumulating unit filled with cold accumulating materials so that theheat energy thereof is supplied to the cold accumulating material. Then,the operating medium expanded here flows in an opposite direction andreceives heat energy from the cold accumulating material. As therecuperation effect is improved in this process, the heat efficiency ofthe operating medium cycle is improved so that a further lowertemperature is realized.

As a cold accumulating material for use in the above-describedrefrigerator, conventionally Cu. Pb and the like have been used.However, these cold accumulating materials have a very small volumetricspecific heat in extremely low temperatures below 20 K. Therefore, theaforementioned recuperation effect is not exerted sufficiently so thatit is difficult to realize the extremely low temperatures.

For the reason, recently to realize temperatures nearer absolute zero,use of magnetic cold accumulating material made of intermetalliccompound formed from a rare earth element and transition metal elementsuch as Er₃ Ni, ErNi, ErNi₂, ErRh, HoCu₂ indicating a large volumetricspecific heat in an extremely low temperature range has been considered.

The aforementioned magnetic cold accumulating material is usuallyprocessed to a sphere of 0.1-0.5 mm in diameter to carry out effectiveheat exchange with the operating medium such as He gas and actually usedin the form of a magnetic particle. By applying the cold accumulatingunit filled with the spherical magnetic particles to the GMrefrigerator, a refrigerating operation to produce an arrival lowesttemperature of 4 K is realized.

FIG. 10 is a sectional view showing an example of a structure of a lowtemperature cold reserving unit 30 using the aforementioned GMrefrigerator 10, specifically a cold reserving unit for asuperconductive magnet constituting a major part of a superconductiveMRI unit, magnetic floating train, superconductive power storage unit(SMES), in-magnetic field single crystal pull-up apparatus and the like.

The low temperature cold reserving unit 30 in FIG.10, comprises asuperconductive magnet 31 which is an object to be cooled, a GMrefrigerator 10 for cooling this superconductive magnet 31 at ultra-lowtemperatures, and a plurality of heat shielding members 32 disposed soas to surround the superconductive magnet 31, these components beingdisposed within the a vacuum container 33. The aforementioned pluralityof the heat shielding members 32 are supported in the vacuum container33 through a supporting member 34. Further, there is provided a heatswitch 35 for thermally cutting off a cooling means such as therefrigerator 10 from an already cooled object.

As the aforementioned heat shielding member 32, a copper (Cu) platehaving a thickness of 1-2 mm is widely used. To suppress invasion ofheat from outside so as to increase cooling efficiency of the overallcold reserving system, the heat shielding members 32 are disposed inmultiple layers.

However, different from the conventional GM refrigerator in which therefrigerating cycle is as low as several Hz, in such a refrigeratorcarrying out high-speed cycle operation like starling refrigerator orpulse tube refrigerator in which the refrigerating cycle is several 10Hz, a pressure loss in the cold accumulating unit filled with theaforementioned spherical magnetic particles increases so that heatexchange between the operating medium and magnetic particles becomesinsufficient. Therefore, the conventional refrigerator has a problem inwhich a sufficient refrigerating capacity cannot be exerted.

On the other hand, as a measure for reducing the pressure loss in theaforementioned cold accumulating unit, there has been used as a trialsuch a method in which the magnetic cold accumulating materials formedin the form of a punched plate or ribbon-shaped plate having a pluralityof through holes are wound in the form of a roll or such a method inwhich mesh-shaped cold accumulating materials are stacked in multiplelayers so as to form a laminated screen.

However, because the aforementioned magnetic cold accumulating materialhas a very high brittleness particular in intermetallic compounds,drilling or bending is very hard, and therefore it is substantiallydifficult to reduce the pressure loss in the cold accumulating unit bythe shape of the cold accumulating material.

On the other hand, if a refrigerator is stopped or low temperatureliquefied gas such as helium (He) is evaporated in the conventional lowtemperature cold reserving unit using copper-made heat shieldingmaterial, the temperature of the heat shielding material rises for ashort time because the specific heat of copper under low temperatures issmall so that an effect of preventing heat invasion from outside islost.

Further, recently, there has been considered a system in which thecooling means is separated from an already cooled object and such anobject to be cooled as the superconductive magnet is operated in acompact condition. However, because the conventional heat shieldingmaterial made of only metallic material such as copper has a smallspecific heat, its cold reservation effect is small so that an object tobe cooled cannot be maintained at low temperatures for a long time.

As a countermeasure for the above mentioned problem, the inventors ofthis invention have considered application of magnetic cold accumulatingmaterial made of intermetallic compound containing rare earth elementsand transition metallic elements indicating a large specific heatparticularly in extremely low temperature range such as Er₃ Ni, ErNi,HoCu₂ to composition material of the heat shielding material. However,because generally the magnetic cold accumulating material is brittle, itis very difficult to process to a sheet-like shape having such a sizewhich is used as the heat shielding material.

A cylindrical heat shielding material as shown in FIG. 10 is preferablefor an object to be cooled such as the superconductive coil, andhowever, processing of the brittle magnetic cold accumulating materialto a cylindrical shape or curved shape is more difficult as compared toprocessing to a flat shape.

On the other hand, the magnetic cold accumulating material made of rareearth elements such as Nd has an inferior specific heat characteristicthan the magnetic cold accumulating material made of the intermetalliccompound. Further, such material has a relatively larger specific heatin extremely low temperature as compared to ordinary metals such as Cuand can be processed to a sheet like shape. However, generally, the heatshielding material is often used in a relatively large area shape andused under a condition in which the heat shielding material is subjectedto application of a large load. However, because the structural strengthof the heat shielding material made of single rare earth element such asNd is insufficient, it cannot be applied to the heat shielding materialwithout any treatment.

The present invention has been achieved to solve the above describedproblems and a first object of the invention is to provide a coldaccumulating material for extremely low temperature cold capable ofexerting a sufficient refrigerating performance with a small pressureloss of refrigerant (operating medium) and easy to process to a shapereducing the pressure loss, and a refrigerator using the same.

A second object of the invention is to provide a heat shielding memberwhich is capable of preventing an invasion of heat effectively, easy toprocess to any shape and has an excellent structural strength.

DISCLOSURE OF THE INVENTION

To achieve the above object, the cold accumulating material forextremely low temperature cold according to the present invention is soformed that pores of a porous carrier thereof are filled with magneticparticles containing a rare earth element. Preferably the porous carrieris composed of a sheet-shaped porous metal or meshed metal. Preferably,the porosity of the porous carrier is 90% or more. Further, preferablythe porous carrier is composed of a foamed metal. Further preferably theporous carrier is formed in the form of a sheet and a plurality ofconvex portions are formed on at least one surface of the porouscarrier.

Preferably, a cold accumulating material for extremely low temperaturecold is formed in such a way that magnetic particles containing a rareearth element are mixed with binding agent, solvent, dispersant andplasticizer so as to prepare uniform slurry and the magnetic particlesare bonded to each other by forming the obtained slurry in the form of asheet. A cold accumulating material for extremely low temperature coldwherein magnetic particles containing rare earth elements are mixed withthe binder, solvent, dispersant and plasticizer so as to prepare uniformslurry and the magnetic particles are bonded to each other by formingthe obtained slurry in the form of a sheet. Further, preferably, aplurality of gas-passing holes are disposed in the sheet-shaped moldedobject comprising the magnetic particles.

The refrigerator according to the present invention includes a coldreserving unit loaded with the extremely low temperature coldaccumulating material formed by filling the pores of the porous carrierwith magnetic particles containing the aforementioned a rare earthelement.

Preferably the cold accumulating material for extremely low temperaturecolds formed in the sheet like shape are loaded in the cold accumulatingunit such that they are wound in the form of a roll. Further, the coldaccumulating material for extremely low temperature cold is formed of aplate-shaped cold accumulating element having a plurality of air passingholes and a plurality of the cold accumulating elements are stacked inmultiple layers in the axial direction of the cold accumulating unit.

The heat shielding material for extremely low temperatures according tothe present invention is formed by bonding the cold accumulatingmaterial for extremely low temperature cold prepared in the above mannerto a reinforcement member made of a different material from this coldaccumulating material for extremely low temperature cold.

Preferably, the aforementioned reinforcement material is made ofmetallic material of at least one kind selected from Cu, Al, Fe, Ni oran alloy constituted mainly of the metallic material. Further, the coldaccumulating material for extremely low temperature cold is asheet-shaped cold accumulating material formed by filling the magneticparticles with the pores of the porous carrier together with the binder.Preferably, the cold accumulating material for extremely low temperaturecold and reinforcement member are bonded to each other with theaforementioned binder.

The magnetic particles for use in the present invention includesmagnetic particles composed of, for example, intermetallic compoundcontaining a rare earth element expressed by

[Expression 1]

general formula:

    RM.sub.z                                                   (1)

(In this expression, R indicates at least one kind of rare earth elementselected from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm andYb and M indicates at least one kind of metallic element selected fromNi, Co, Cu, Ag, Al, Ru, In and Rh and z indicates a number in a range of0.01-9.0 in atomic ratio. The same meaning is held in the sameexpression which comes later) and single rare earth element such as Nd.The aforementioned magnetic particle can be prepared by mechanicallycrushing a mother alloy of a predetermined composition. Further, it maybe prepared by processing molten metal containing a rare earth elementof a predetermined amount or a molten rare earth element by such a rapidcooling method as centrifugal spray method, rotary disc method (RDPmethod), inert gas atomizing method, single roll method and double rollmethod. The shape of the aforementioned magnetic particle may be anyshape such as irregular shape or spherical shape.

If a diameter of the magnetic particle exceeds 5 mm, the loading(packing) characteristic to the porous carrier is deteriorated.Therefore, the particle diameter of the magnetic particle is preferredto be less than 5 mm, more preferably the particle diameter thereof isless than 1 mm and more preferably, the particle diameter is less than0.2 mm.

Preferably, the porous carrier in which the magnetic particles are to beloaded is formed of Ni, Cu, Pb, Fe, stainless steel, Ni alloy, Cu alloy,Pb alloy or Fe alloy which has an excellent processing characteristicand is cheap. Further, the surface of a main body formed of these metalsor alloys may be plated with Cr.

The aforementioned porous carrier includes porous metal such as foamedmetal and meshed metal formed by weaving metallic wires horizontally andvertically.

The aforementioned porous carrier can be produced according to thefollowing process. That is, the foamed resin containing continuous airbubbles such as urethane is subjected to conductive treatment, theexternal surface of the foamed resin is electrically plated withmetallic components such as Ni, Ni--Cr, Ni--Al, then, this is subjectedto heat treatment so as to evaporate resin component and at the sametime, subjected to alloying treatment. As a result, the resin parts areconverted to pores so that a carrier material of porous metal can beobtained. By processing this carrier material to a sheet-like shape orblock-like shape, the porous carrier for use in the present inventioncan be obtained.

A larger advantage is gained as the porosity of the porous carrier ishigher, because a larger amount of magnetic particles having a largevolumetric specific heat is loaded under extremely low temperatures. Theporosity of the porous carrier is preferred to be more than 20 vol. %and more preferably more than 60 vol. % and further preferably more than90 vol. %. The porosity of the aforementioned porous carrier can beadjusted freely in a range of 10-98 vol. % by controlling the foamdegree of the foam resin in the production process.

The porous carrier prepared as described above can be loaded with alarge amount of magnetic particles in parallel to an increase of theporosity. Further because it has a high specific surface area and allthe pores communicate, there is produced little air passing resistanceso that the pressure loss is very small.

The cold accumulating material for extremely low temperature coldaccording to the present invention is formed by loading the pores of theporous carrier prepared in the above manner with magnetic particles. Inthe case when heat exchange between the operating medium and coldreserving material is applied in the cold reserving unit designed forthe operating medium passing through an interior of the sheet like coldreserving material to exchange heat directly with the cold reservingmaterial, the ratio (filling rate) of loading the porous carrier withmagnetic particles is designed to be 20-90%. If the filling rate is lessthan 20%, the cold accumulating effect by the magnetic particles is notsufficient although the flow (gas passing) resistance of the operatingmedium is small. On the other hand, if the filling rate exceeds 90%, theflow resistance of the operating medium increases excessively so thatthe pressure loss in the cold accumulating unit is increased thereby thecold accumulating effect being reduced.

The filling rate mentioned herein refers to a rate of a volume of themagnetic cold accumulating particles occupying an entire volume(including the porous carrier) of the sheet-shaped cold accumulatingmaterial the thickness of which has been finally adjusted by rollingwork which will be described later.

In using a cold accumulating unit (gap cold accumulating unit) in whichheat exchange between the operating medium and cold accumulatingmaterial is not carried by the operating medium passing through aninterior of the sheet-shaped cold accumulating material but by theoperating medium passing over the surface of a sheet like coldaccumulating material having a low flow path resistance with mainly thecold accumulating material, the filling rate of the magnetic particlesis 60-92%. Preferably, the filling rate is 65-88% and more preferably70-85%. If the filling rate is too small, the cold accumulating effectby the magnetic particles is reduced, and if the filling rate is toolarge, the magnetic particle is distorted by a stress applied upon thefilling operation so that the characteristic of the magnetic particle isdeteriorated.

By adhering thermoplastic resin such as polyvinyl alcohol (PVA) orthermal hardening resin such as epoxy resin and polyimide to the surfaceof the porous carrier or magnetic particle as binder, the couplingstrength between the magnetic particle and porous carrier can beintensified, so that a possibility that the magnetic particles drop by avibration or the like is eliminated. As a result, a cold accumulatingmaterial for extremely low temperature cold excellent in structuralstrength can be obtained.

As a method for producing a complex structure by filling the porouscarrier with the magnetic particles, a following method can be applied.That is, slurry-like paste is prepared by mixing binder and solvent withmagnetic particles prepared by the aforementioned molten metal rapidlycooling method or mechanical crushing method and this paste is equallyloaded (uniformly packed) in the porous carrier of porous metal ormeshed metal prepared in the above manner and dried in reduced pressureenvironment at 100-140° C. for 0.5-2.0 hours so as to remove solventcomponent.

By pressing or rolling the porous carrier filled with the magneticparticles (powder), the coupling strength between the magnetic particlesand porous carrier is intensified and the thickness of the coldaccumulating material for extremely low temperature cold formed in thesheet-like shape can be adjusted.

The thickness of the aforementioned sheet-shaped cold accumulatingmaterial for extremely low temperature cold is in a range of 0.01-2 mmso as to secure an easiness of processing to a predetermined shape bybending. The thickness is preferred to be 0.05-1.0 mm and further it ismore preferred to be 0.1-0.5 mm.

By rolling the porous carrier filled with the magnetic particles bymeans of an embossing roll having an uneven surface, a plurality ofconvex portions can be formed on a surface of the porous carrier. Whenthis porous carrier having the convex portions is wound so as to form acylindrical cold accumulating material for extremely low temperaturecold, the adjacent porous carriers do not closely contact to each otherbut are isolated by the convex portions. Thus, in a case when thisporous carrier is applied to the aforementioned gap cold accumulatingunit, the operating medium (refrigerant) such as He gas can becirculated smoothly through spaces formed by this isolation so that thepressure loss of the operating medium can be reduced.

Further, as a transition metal or various alloys for a compositionmaterial of the porous carrier such as the aforementioned porous metaland meshed metal, it is possible to select those having a higher heatconduction (thermal conductivity) under low temperatures than themagnetic cold accumulating material which can be expressed by theaforementioned general formula of RM_(z). Even if the cold accumulatingmaterial for extremely low temperature cold of the present inventioncomposed of the aforementioned porous carrier and magnetic particles isused in the starling refrigerator or pulse tube refrigerator forcarrying out high-speed cycle operation reducing heat penetration depth,heat transfer action by the porous metal or meshed metal is exertedsufficiently to the magnetic particles loaded in deep portions of theporous carrier so that heat transfer between the magnetic particles,carrier and operating medium is exerted rapidly.

On the other hand, there may occur a case when heat conduction from hightemperature side of the cold accumulating unit to low temperature sidethereof is required depending on a design of the cold accumulating unit.In such a case, as a transition metal or various alloys for composingthe porous carrier, conversely with the above, it is favorable toutilize a material having a low heat conduction in a low temperaturerange like stainless. In this selection, any case can be adopteddepending on a design of the refrigerator or cold accumulating unit.

When the porous carrier filled with the aforementioned magneticparticles is loaded in the cold accumulating unit, it is possible towind the porous carrier in the form of a roll or cut the sheet in anappropriate shape. If it is processed in the form of a roll, it ispossible to stack a plurality of the carriers having a small width andload them in the cold accumulating unit. When the sheet-shaped carrieris used, two cases can be considered, that is, a case when it is loadedwith its surface being substantially parallel to a flow direction of theoperating medium and a case when it is loaded with the surface beingperpendicular to the flow direction. If it is loaded so that it isperpendicular to the flow direction, the surface needs to be drilled ora material having a low filling density of the magnetic particles needsto be used to secure a flow path of the operating medium.

The cold accumulating material for extremely low temperature cold havingthe aforementioned structure is formed by filling the pores of theporous carrier having a low air passing resistance and excellentprocessing characteristics and the magnetic particles having a highbrittleness are held by a porous carrier easy to deform. Therefore, themagnetic particles are easy to process to a shape reducing the pressureloss. Therefore, even if this cold accumulating material is used as acold accumulating material of the cold accumulating unit carrying outhigh-speed cycle operation such as the starling refrigerator and pulsetube refrigerator, an operation having a low pressure loss and a highheat exchange efficiency is enabled so that a refrigerator having a highrefrigerating capacity can be achieved.

Further, it is possible to produce a sheet-shaped or plate-shaped coldaccumulating material for extremely low temperature cold by formingcrushed powder of the aforementioned magnetic particles to a sheet formor plate form in various methods. That is, the magnetic particlesprepared in the above method are crushed to have an average diameter ofseveral μm (the number of the particles of which diameter is less than50 μm is more than 70%) and then binder (binding agent), solvent,dispersant, plasticizer are added to the obtained powder as required andmixed uniformly so as to prepare a slurry.

Although the aforementioned binder is not restricted to a particularone, polyacrylate, polymetacrylate, cellulose acetate, polyvinylbutyral, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), methylcellulose, polyethylene glycols, carboxymethyl cellulose and the likecan be used as the binder. The solvent which can be used includesacetone, toluene, trichloroethylene, ethyl alcohol, ethyl acetate, waterand the like. The dispersant which can be used includes glyceroltriolate, allyl sulfonic acid, phosphates and various surface activeagents (surfactants). The plasticizer to be added to improve theflexibility and processing characteristic of the molded object includesoctyl phthalate, butyl benzyl phthalate, glycerin, polyethylene glycols,sucrose acetate isobutylate, di-butyl phthalate, di-isodecyl phthalateand the like.

Then, thus obtained slurry is coated on, for example, a metallic plateor plastic film surface to be formed in the form of a sheet or theslurry is formed in the form of a plate. Although the forming method isnot restricted to a particular one, doctor blade method, roll formingmethod, gravure coating method and the like can be used. The coldaccumulating material formed in the form of a sheet or plate issubjected to heat treatment as required so as to evaporate binder andsolvent and dry the body.

Different from the conventional cold accumulating material having a highbrittleness, the sheet like cold accumulating material formed asdescribed above can be deformed to various shapes. For example, if thesheet-shaped cold accumulating material is wound in the form of a rolland loaded in the cold accumulating unit of a refrigerator, it can bemade to function as a cold accumulating material for extremely lowtemperature cold having a small gas-passing resistance. Then, bychanging the winding style of the aforementioned sheet-shaped coldaccumulating material, its gas-passing resistance can be changed freely.Particularly because the shape of the sheet-shaped cold accumulatingmaterial can be changed so as to reduce the gas-passing resistance, itcan be used effectively as a cold accumulating material of arefrigerator carrying out high-speed operation.

Further, it is permissible to dry the aforementioned slurry directlywithout being coated on anything, form it by press to a plate and use itas a plate-shaped cold accumulating material. That is, if a plurality ofgas-passing holes are made in a direction of the thickness of theplate-shaped cold accumulating material and the plate-shaped coldaccumulating materials are stacked in multiple layers in the coldaccumulating unit through fine spacers, a cold accumulating materialhaving an equalized flow of refrigerant (He gas) and small gas-passingresistance can be obtained.

Although the cross section of the aforementioned gas passing hole is notrestricted to any particular shape, a circular shape which facilitatesdrilling work is preferable. In this case, although the diameter of thegas passing hole is in a range of 10 μm-1 mm, a range of 20 μm-300 μm isparticularly preferable. Even if the cross section of the gas passinghole is other case than the circular shape, it is favorable to have across section equivalent to the circular shape. Although an interval ofarrangement of the aforementioned gas passing holes is set to 20 μm-2mm, a range of 30-400 μm is further preferable. A thickness of theplate-shaped cold accumulating material is preferred to be 0.5-5 mm.

In a case when the cold accumulating material wound in the form of aroll is accommodated in the cold accumulating unit, He gas flows as therefrigerant concentratedly in the gap near a center of the roll, so thatthe overall flow of the refrigerant is inclined to be unequal. However,in a case when the plate-shaped porous cold accumulating material arestacked in multiple layers, the flow of the refrigerant becomes equal sothat the refrigerating effect can be further raised. The gas passingresistance can be adjusted arbitrarily by changing the diameter anddisposing pitch of the gas passing holes. When the aforementionedplate-shaped porous cold accumulating materials are stacked in multiplelayers, as compared to a case when the conventional spherical magneticparticles are loaded in the same loading rate, the gas passingresistance can be further reduced, so that a higher speed cycleoperation of the refrigerator can be achieved.

On the other hand, the heat shielding material according to the presentinvention is formed by bonding the cold accumulating material forextremely low temperature cold prepared in the above manner to thereinforcement member made of a different material from this coldaccumulating material for extremely low temperature cold.

The aforementioned extremely low temperature cold accumulating materialis prepared in the following manner. First, binding agent (binder),solvent and the like are mixed with the magnetic cold accumulatingmaterial powder obtained by crushing the magnetic powder having theaforementioned composition so as to prepare a slurry. After that, thusobtained slurry is loaded in the pores of the porous carrier and thensolvent components are vaporized so as to form the sheet-shaped coldaccumulating material.

Here, heating or evacuating is effective to vaporize the solventcomponents. As the porous carrier, it is possible to use a meshed metalcomposed of fiber metal as well as porous metal such as foam metal. As acomposition material of the porous carrier, such metallic material asNi, Cu, Pb, Fe, Al, Ni alloy, Cu alloy, Pb alloy, Fe alloy, Al alloy andstainless steel is preferable.

The porosity of the porous metal or meshed metal is more advantageous asit is higher because more magnetic cold accumulating material having ahigh specific heat can be loaded therein. Although the porosity is setto more than 20 vol. %, it is favorable that it is more than 60 vol. %,and more than 85 vol. % is further preferable.

The transition metal or alloy composing the aforementioned porous metalor meshed metal has a higher heat conduction in low temperatures thanordinary metallic materials. Therefore, in a case when an object to becooled is cooled by heat transferred from the refrigerator, the heatconduction efficiency of the porous metal or meshed metal is high sothat the magnetic cold accumulating material loaded in the sheet likecold accumulating material can be cooled effectively.

Although the binding agent (binder) for binding the aforementionedmagnetic cold accumulating material powder with the porous carrier isnot restricted, thermoplastic resin such as polyvinyl alcohol (PVA),carboxymethyl cellulose (CMC) and thermosetting resin such as epoxyresin and polyimide can be preferably used.

As the cold accumulating material for extremely low temperature cold, itis possible to use a cold accumulating material prepared by the moltenmetal rapidly cooling method mentioned below or a cut or rolled coldaccumulating material. Further it is possible to use cold accumulatingmaterial processed in the form of thin piece (flake), needle, or powderby processing the molten metal melted under a predetermined compositionby such molten metal rapidly cooling method as single roll method,double roll method and centrifugal spray method. In this case, becausethe thickness of the thin piece or the diameter of needle-shaped orpowder-shaped cold accumulating material is less than 0.4 mm, pluralthin pieces or the like can be bonded to each other in the thicknessdirection using adhesive agent (binding agent). Further, the magneticcold accumulating material composed of only such rare earth element asNd processed by cutting its ingot or rolling the material in the form ofa plate can be also used.

Because generally the magnetic cold accumulating material composed ofparticularly the intermetallic compound is of brittle material, it isdifficult to process to a sheet in industrial scale. However, if themagnetic cold accumulating material is formed in the form of arelatively small area plate or a chip, it can be produced by a method ofcutting its ingot or a method of crushing the ingot and sintering thecrushed powder.

In this case, an area of each magnetic cold accumulating material ispreferred to be in a range of 1-1000 cm². A large area plate likemagnetic cold accumulating material in which this area exceeds 1000 cm²is difficult to process and has a small mechanical strength, it isliable to be damaged in a process of assembly to the heat shieldingmember or during an operation. On the other hand, if an object to becooled having a large area is covered with a plate less than 1 cm² or achip like magnetic cold accumulating material, the joint betweenadjacent magnetic cold accumulating materials increases thereby the heatshielding effect being reduced. Therefore, although the area of eachmagnetic cold accumulating material is in a range of 1-1000 cm², 2-500cm² is more preferable and 3-100 cm² is further preferable. Preferably,the thickness of the respective magnetic cold accumulating material is0.5-50 mm.

The reinforcement member to which the aforementioned magnetic coldaccumulating material is to be bonded has a function for supporting andreinforcing a magnetic cold accumulating material which cannot beprocessed to a large configuration or a magnetic cold accumulatingmaterial having no sufficient structural strength as well as the heatshielding effect. As the composition material of the aforementionedreinforcement member, not only such metallic materials as Ni, Cu, Fe,Al, Ni alloy, Cu alloy, Fe alloy, Al alloy, stainless steel or the likebut also the epoxy resin and fiber reinforced plastic (FRP) can be used.Among the above mentioned composition materials, particularly Cu, Al, Cualloy and Al alloy are preferable for the reason that their heatconductions are high. Further, Fe base metallic material such asstainless steel is preferable in viewpoints of cheap cost.

The extremely low temperature heat shielding member of the presentinvention is formed by bonding each of various magnetic coldaccumulating materials to the aforementioned reinforcement memberintegrally. Here, the binding agent (binder) used for bonding themagnetic particles to the porous carrier can be used as adhesive agentfor bonding the sheet-shaped magnetic material in which the pores of theporous carrier is filled with the magnetic particles to thereinforcement member.

That is, by bringing the sheet-shaped magnetic cold accumulatingmaterial into a contact with the reinforcement member and fixing itthereon before the pores of the porous carrier is filled with a slurryprepared by mixing the magnetic particles with binding agent, solventand the like and the solvent components are vaporized, the sheet-shapedmagnetic cold accumulating material can be integrally bonded to thereinforcement member by the binding agent (binder) which binds themagnetic particles with the porous carrier.

Further, to intensify close-contacting property between the magneticcold accumulating material and reinforcement member, reduce heatresistance and increase the bonding strength between the both members,screwing the magnetic cold accumulating material to the reinforcementmember or binding the reinforcement member to an external surface of themagnetic cold accumulating material using a belt or wire is alsoeffective.

The heat shielding member having the above-described structure can beeasily processed to any shape and this heat shielding member is capableof maintaining an object to be cooled at low temperatures for a longtime. Particularly, this heat shielding member is capable of improvingthe temperature stability of an apparatus operated in an extremely lowtemperature range such as superconductive wire, superconductive deviceand infrared ray sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a cutout section of a coldaccumulating unit filled with cold accumulating material for extremelylow temperature cold according to the present invention,

FIG. 2 is a diagram showing a particle structure of the coldaccumulating material for extremely low temperature cold according tothe present invention and an enlarged view of a portion II of FIG. 1,

FIG. 3 is a perspective view showing other example of the coldaccumulating material for extremely low temperature cold according tothe present invention,

FIG. 4 is a perspective view showing a cutout section of a coldaccumulating unit filled with the cold accumulating material forextremely low temperature cold according to other example of the presentinvention,

FIG. 5 is a sectional view showing an example of a heat shielding memberaccording to the present invention,

FIG. 6 is a perspective view with a section showing other example of theheat shielding member according to the present invention,

FIG. 7 is a perspective view with a section showing other example of theheat shielding member according to the present invention,

FIG. 8 is a perspective view showing other example of the heat shieldingmember according to the present invention,

FIG. 9 is a sectional view showing major parts of a GM refrigerator,

FIG. 10 is a sectional view showing an example of a structure of a coldreservation unit using the GM refrigerator and heat shielding member and

FIG. 11 is an enlarged sectional view of a portion XI in FIG. 10.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the embodiment of the present invention will be described moreconcretely with reference to examples mentioned below.

EXAMPLE 1

First, ErNi mother alloy was produced by high frequency melting method.Next, this ErNi mother alloy was mechanically crushed so as to obtainalloy particles of less than 200 meshes. Water solution in whichpolyvinyl alcohol was dissolved in 4 wt % as a binding material wasadded to the obtained ErNi alloy particles in a rate of 25% relative tothe alloy powder weight and uniformly agitated so as to prepare aslurry-like paste.

On the other hand, a plurality of Ni made porous carriers (product name:Cerumetto, made by Sumitomo Electric Industries ) of 1.6 mm inthickness, 50 mm in width, 400 mm in length having a porosity of 95 vol.% were prepared.

Next, the pores of the aforementioned Ni made porous carrier are equallyfilled with the slurry-like paste prepared in the above manner and thendried at 120° C. in the reduce pressure atmosphere (1-100 Torr) for anhour so as to evaporate water. As a result, a sheet-shaped coldaccumulating material in which the ErNi magnetic particles adhered tothe porous carrier through the binding agent was prepared.

Next, the obtained sheet-shaped cold accumulating material was rolledusing an embossing roll having an uneven surface and consequently, asheet-shaped cold accumulating material for extremely low temperaturecold having a thickness of 0.8 mm according to the Example 1 wasprepared.

The sheet-shaped cold accumulating material for extremely lowtemperature cold 1 according to the Example 1 has a compositionstructure in which the pores 3 of the Ni made porous carrier 2 is filledwith a plurality of magnetic particles (ErNi alloy powder) asschematically shown in FIG. 2. Each of the magnetic powders 4 is firmlybonded to the porous carrier 2 through polyvinyl alcohol as a bindingagent. Further, a bump of 0.05 mm in height (not shown) is formed on thesurface of the sheet-shaped cold accumulating material for extremely lowtemperature cold 1 by rolling by means of the embossing roll.

Next, ends of the obtained sheet-shaped cold accumulating materials forextremely low temperature cold 1 were spot-welded to join pluralsheet-shaped cold accumulating materials thereby producing a continuousribbon-shaped cold accumulating material having a width of 50 mm. Then,thus obtained ribbon-shaped cold accumulating material was wound so asto obtain a roll-shaped cold accumulating material for extremely lowtemperature cold. In this roll-shaped cold accumulating material, anadjacent sheet-shaped cold accumulating material is isolated from theother by the bump (protrusion) formed on the surface thereof.

Then, the above-described roll-shaped cold accumulating materials forextremely low temperature cold 1 were loaded in a cold accumulating unit5 having an internal diameter of 25 mm and a height of 50 mm as shown inFIG. 1. The loading ratio of the magnetic powder 4 in this coldaccumulating unit 5 was 73 vol %. This cold accumulating unit 5 was usedas a third cold accumulating unit of a three-stage pulse tuberefrigerator and operated at an operating frequency of 10 Hz. As aresult, a refrigerating capacity of 0.14 W at 10 K was obtained.

COMPARATIVE EXAMPLE 1

On the other hand, ErNi alloy prepared at the Example 1 was melted andthe obtained molten metal was diffused by centrifugal spray method.Immediately after that, it was cooled rapidly and solidified so as toproduce spherical magnetic particles. The obtained magnetic particleswere sieved and spherical magnetic particles having a diameter of0.15-0.18 mm were selected. Next, the selected magnetic particles wereloaded in the cold accumulating unit 5 (25 mm in internal diameter×50 mmin height) used in the Example 1 shown in FIG. 1. The loading rate ofthe magnetic powder 4 in this cold accumulating unit 5 was 62 vol %.

Then, the cold accumulating unit 5 filled with the spherical magneticparticles was used as the third cold accumulating unit of the pulse tuberefrigerator like the Example 1 and operated under the same conditions.As a result, the no-load temperature did not reach 10 K but was 16 K.That is, a sufficient refrigerating capacity could not be obtained.

EXAMPLE 2

FIG. 3 is a perspective view showing a shape and structure of the coldaccumulating material for extremely low temperature cold 1a according tothe Example 2. This cold accumulating material for extremely lowtemperature cold 1a was produced by bending the sheet-shaped coldaccumulating material prepared in the Example 1 at intervals in thelength direction thereof by means of a press so as to create a pluralityof convex portions 6. By winding the bent sheet-shaped cold accumulatingmaterial as shown in FIG. 3, a sheet-shaped cold accumulating materialadjacent in the radius direction is separated by the convex portion 6thereby reducing a gas flow resistance in the width direction of thesheet-shaped cold accumulating material.

Like the Example 1, the cold accumulating material for extremely lowtemperature cold 1a according to the Example 2 was loaded in the coldaccumulating unit 5 shown in FIG. 1, used as the third cold accumulatingunit of the pulse tube refrigerator and operated under the sameconditions. As a result, a refrigerating capacity of 0.11 W at 10 K wasobtained.

EXAMPLE 3

In the Example 1, ErNi magnetic particles were loaded in Ni made porouscarrier having a thickness of 2.0 mm and a diameter of 25 mm andporosity of 97% so as to prepare a sheet-shaped cold accumulatingmaterial. A plurality of holes each having a diameter of 0.2 mm weredrilled at an interval of 0.5 mm in this sheet-shaped cold accumulatingmaterial. This cold accumulating material was loaded in the coldaccumulating unit like the Example 1 and operated as the third coldaccumulating unit of the pulse tube refrigerator under the sameconditions. As a result, a refrigerating capacity of 0.13 W at 10 K wasobtained.

In the cold accumulating materials for extremely low temperature cold 1,1a according to the aforementioned respective embodiments, the operatingmedium (refrigerant) such as He gas passes through a flow path having alow gas-passing resistance between adjacent sheet-shaped coldaccumulating materials so as to carry out heat exchange on the surfaceof the sheet-shaped cold accumulating material. Therefore, even whenhigh speed cycle operation is carried out, the pressure loss is smalland heat exchange rate is high, so that it has been made evident that anexcellent refrigerating capacity can be exerted.

Particularly, even if hard processing such as forming a coil or roll bywinding is carried out, the porous carrier is freely deformed so thatthe magnetic particles are seldom broken or damaged. That is, it ispossible to create to a shape capable of reducing the pressure loss.

Comparing the Example 1 with Comparative Example, in the case of theExample 1, the loading rate of the magnetic particles relative to thecold accumulating unit could be raised without increasing the pressureloss, and therefore, it is considered that a large difference wasproduced in the refrigerating capacity.

Although according to the above described embodiment, the sheet likecold accumulating material for extremely low temperature cold wascreated by loading the magnetic particles in the porous carrier therebyproducing complex structure, it is also possible to form thesheet-shaped cold accumulating material for extremely low temperaturecold by combining the carrier and magnetic particles in a followingmethod.

That is, it is possible to nip or clamp the magnetic particles betweenthe mild metallic sheets of Pb or the like and press it so as to combinethem integrally and form the sheet-shaped cold accumulating material forextremely low temperature cold.

Further, it is permissible to carry out so-called canning which meansevacuating and sealing after magnetic particles are loaded in a bagmember formed of metallic material such as Ni, Cu, Pb, Al and roll thecanned sheet like bag so as to integrally combine the metallic materialand magnetic particles and then form the sheet-shaped cold accumulatingmaterial for extremely low temperature cold.

Further, it is also permissible to pulverize the magnetic coldaccumulating material to fine particles having a diameter of several μm,add binder and solvent to thus obtained magnetic powder to prepare aslurry, form a sheet-shaped molded form from this slurry according to,for example, doctor blade method or roll forming method and heat thissheet-shaped molded form to evaporate the binder component therebyobtaining the sheet-shaped magnetic cold accumulating material. Further,it is also possible to create a sheet-shaped cold accumulating materialhaving a little gas flow resistance by drilling the above mentionedsheet like molded form.

Further, it is also possible to coat the surface of the magneticparticles with a metal having a low melting point and not reacting withthe magnetic particle-like Pb by mechanical alloying method, forming thecoated magnetic particles in the form of a sheet, subjecting theobtained formed object to heat treatment to melt the low melting pointmetal and couple the magnetic particles with each other by this lowmelting point metal thereby producing the sheet-shaped magnetic coldaccumulating material.

Next, the plate like cold accumulating material for extremely lowtemperature cold produced by molding pulverized magnetic particles willbe described with reference to the following examples.

EXAMPLE 4

A mother alloy of magnetic material having such a composition as HoCu₂was prepared by high frequency melting method. This mother alloy wascrushed by using jaw crusher, hammer mill, and ball mill successively soas to prepare magnetic alloy powder having an average particle diameterof 10 μm. Relative to alloy powder weight, acrylic resin of 7 wt % asbinder, methyl isobutyl ketone (MIBK) of 70 wt % as solvent and di-butylphthalate of 2.8 wt % as plasticizing agent were added to this magneticalloy powder and mixed with alumina ball by a pot roller for 24 hours soas to prepare a uniform slurry.

After the obtained slurry was dried, it was sieved with #60 sieve so asto adjust the particle diameter. This dried powder was loaded in a moldand molded by a molding pressure of 180 kg/cm² so as to produce a platelike cold accumulating material having a diameter of 28 mm and athickness of 1 mm. Further, as shown in FIG. 4, gas passing holes(through hole) of 100 μm in diameter were mechanically drilled at apitch of 200 μm in this plate-shaped cold accumulating material.Further, the obtained cold accumulating material plate having aplurality of the holes was degreased in nitrogen gas environment at 700°C. for two hours so as to produce a plate-shaped porous coldaccumulating material 1b.

Then, thus obtained 50 pieces of the plate-shaped porous coldaccumulating materials 1b were stacked in multi-layers in the axialdirection of the second stage cold accumulating unit 5a of the 2-stageexpansion type pulse tube refrigerator as shown in FIG. 4 with a teflonmade mesh material inserted in each gap between the plates as a spacerin a process of assembly of a refrigerator. This refrigerator wasoperated at the frequency of 20 Hz. As a result, the no-load temperaturereached 4.0 K and an excellent refrigerating capacity was obtained.

COMPARATIVE EXAMPLE 2

The mother alloy (HoCu₂) prepared in the Example 4 was melted and theobtained molten metal was dispersed and rapidly cooled and solidified bycentrifugal spray method (RDP) so as to prepare spherical magneticparticles. Thus obtained magnetic particles were sieved to selectspherical magnetic particles having diameter of 0.15-0.18 mm. Themagnetic particles were loaded in the second stage cold accumulatingunit 5a of the pulse tube refrigerator used in the Example 4 and arefrigeration test was carried out under the same conditions as theExample 4. The no-load temperature was 13.2 K.

EXAMPLE 5

A mother alloy of magnetic material having such a composition as Er₃ Niwas prepared by high frequency melting method. This mother alloy wascrushed by using jaw crusher, hammer mill, and ball mill successively soas to prepare magnetic alloy powder having an average particle diameterof 8 μm. Relative to alloy powder weight, acrylic resin of 6 wt % asbinder, methyl isobutyl ketone (MIBK) of 70 wt % as solvent and di-butylphthalate of 2.5 wt % as plasticizing agent were added to this magneticalloy powder and mixed with alumina ball by a pot roller for 24 hours soas to prepare a uniform slurry.

Next, the obtained slurry was formed according to doctor blade method soas to prepare a long sheet-shaped cold accumulating material having awidth of 60 mm and a thickness of 300 μm.

Next, gas passing holes (through hole) of 200 μm in diameter weremechanically drilled at a pitch of 300 μm in this sheet-shaped coldaccumulating material. Thus obtained porous sheet-shaped coldaccumulating material was wound so as to obtain a roll-shaped coldaccumulating material having a diameter of 28 mm and a height of 60 mm,and then degreased in nitrogen gas environment at 700° C. for two hoursso as to produce an cold accumulating material for extremely lowtemperature cold according to the Example 5.

Such a roll-shaped cold accumulating material for extremely lowtemperature cold according to the Example 5 was loaded in the secondstage cold accumulating unit of the two-stage expansion type pulse tuberefrigerator in a process of assembly of a refrigerator. Then, thisrefrigerator was operated at the frequency of 20 Hz. As a result, theno-load temperature reached 4.5 K and an excellent refrigeratingcapacity was obtained.

COMPARATIVE EXAMPLE 3

The mother alloy (Er₃ Ni) prepared in the Example 5 was melted and theobtained molten metal was dispersed, rapidly cooled and solidified bycentrifugal spray method (RDP) so as to prepare spherical magneticparticles. The obtained magnetic particles were sieved to selectspherical magnetic particles having diameter of 0.15-0.18 mm. Themagnetic particles were loaded in the second stage cold accumulatingunit of the pulse tube refrigerator used in the Example 5 and therefrigeration test was carried out under the same conditions as theExample 5. As a result, the no-load temperature was 17.0 K.

EXAMPLE 6

Nd molten metal was dispersed and cooled rapidly according to Ar gasatomizing method so as to prepare magnetic powder. The obtained powderwas sieved to select particles having a particle diameter of less than100 μm. Water solution in which polyvinyl alcohol was dissolved in 2 wt% concentration as binder was added to the obtained Nd powder 20% perpowder weight and kneaded uniformly so as to prepare slurry-like paste.

On the other hand, a plurality of Ni made porous carriers (product name:Cerumetto, made by Sumitomo Electric Industries) of 1.6 mm in thickness,50 mm in width, 400 mm in length having a porosity of 95 vol. % wereprepared.

Next, the pores of the aforementioned Ni made porous carrier are equallyfilled with the slurry-like paste prepared in the above manner and thendried at 120° C. in the reduce pressure atmosphere (1-100 Torr) for anhour so as to evaporate water. As a result, a sheet-shaped coldaccumulating material in which the Nd magnetic particles adhere to theporous carrier through the binding agent was prepared.

Next, the obtained sheet-shaped cold accumulating material was rolledusing an embossing roll having an uneven surface and consequently, asheet-shaped cold accumulating material for extremely low temperaturecold having a thickness of 0.8 mm according to the Example 6 wasprepared.

A bump of 0.05 mm in height is formed on the surface of the sheet-shapedcold accumulating material for extremely low temperature cold accordingto the Example 6 by rolling by means of the embossing roll.

Next, ends of the obtained sheet-shaped cold accumulating materials forextremely low temperature cold 1 were spot-welded to join pluralsheet-shaped cold accumulating materials thereby producing aribbon-shaped cold accumulating material having a width of 50 mm. Then,the obtained ribbon-shaped cold accumulating material was wound so as toobtain a roll-shaped cold accumulating material for extremely lowtemperature cold. In this roll-shaped cold accumulating material, anadjacent sheet-shaped cold accumulating material is isolated from theother by the bump (protrusion) formed on the surface thereof.

Then, the above described roll-shaped cold accumulating material forextremely low temperature cold was loaded in a cold accumulating unithaving an internal diameter of 25 mm and a height of 50 mm. This coldaccumulating unit was used as the second stage cold accumulating unit ofthe two-stage pulse tube refrigerator and operated at an operatingfrequency of 20 Hz. As a result, the no-load temperature was 6.3 K andan excellent refrigerating capacity was obtained.

COMPARATIVE EXAMPLE 4

On the other hand, a round bar having a diameter of 50 mm and a heightof 300 mm was produced using Nd prepared in the Example 6. Nd moltenmetal was dispersed according to rotational electrode process (REP) inwhich the obtained Nd round bar was used as an electrode and melted, atthe same time, cooled rapidly and solidified so as to prepare sphericalmagnetic particles. The obtained magnetic particle groups were sieved soas to select spherical magnetic particles having a diameter of 0.15-0.18mm. Next, the selected magnetic particles were loaded in the coldaccumulating unit (25 mm in internal diameter×50 mm in height) used inthe Example 6.

Then, the cold accumulating unit filled with the spherical magneticparticles was used as the second cold accumulating unit of the pulsetube refrigerator as like the Example 6 and operated under the samecondition. As a result, the no-load temperature did not reach 6.3 K butwas 18.2 K. Therefore, a sufficient refrigerating capacity could not beobtained.

Next, the heat shielding member for extremely low temperature coldaccording to the present invention will be described with reference tothe following examples.

EXAMPLE 7

HoCu₂ mother alloy was produced by high frequency melting method. Next,this HoCu₂ mother alloy was mechanically crushed so as to obtain alloypowder of less than 200 mesh. Next, water solution in which polyvinylalcohol was dissolved in 4 wt % concentration as binder was added to theobtained HoCu₂ alloy powder in a rate of 25% relative to alloy powderweight and kneaded uniformly so as to prepare a slurry-like paste. Onthe other hand, a plurality of Ni made porous carriers (product name:Cerumetto, made by Sumitomo Electric Industries) having a thickness of1.6 mm, width of 50 mm and length of 400 mm and porosity of 95 vol. %were prepared.

Next, the pores of the above described Ni made porous carrier wasequally filled with the prepared slurry-like paste so as to prepare asheet-shaped cold accumulating material 36a.

On the other hand, as shown in FIG. 5, a bottomed cylindricalreinforcement member (for a first layer) 37a having a diameter of 200mm×height of 300 mm and a reinforcement member 37b (for a second layer)having a diameter of 230 mm and height of 350 mm, both being made of Cumaterial having a thickness of 1 mm were prepared. Before the binder ofthe aforementioned sheet-shaped cold accumulating material 36a wasdried, the sheet-shaped cold accumulating agent 36a was bonded to anexternal surface of the respective reinforcement members 37a, 37b. Thatis, an excessive amount of the aforementioned paste was coated betweenthe respective sheet-shaped cold accumulating material 36a and thereinforcement members 37a, 37b made of Cu such that it exuded throughthe Ni made porous carrier and the sheet-shaped cold accumulatingmaterial 36a was bonded with the reinforcement members 37a, 37b made ofCu by adhesive force of polyvinyl alcohol which was a binding componentin the paste.

Further, to improve the heat transfer between the sheet-shaped coldaccumulating material 36a and the reinforcement members 37a, 37b, thesheet-shaped cold accumulating member 36a was fixed by screws 38. Afterthat, this was dried in reduced pressure atmosphere at 120° C. for anhour and consequently, as shown in FIG. 5, heat shielding members 39a,39b in which the sheet-shaped cold accumulating member 36a andreinforcement members 37a, 37b were bonded to each other were prepared.

On the other hand, magnetic particles made of Er₃ Ni instead of theaforementioned HoCu₂ was loaded in the pores of the Ni made porouscarrier so as to prepare the sheet-shaped cold accumulating material 36bas shown in FIG. 5. Further, as shown in FIG. 5, a bottomed cylindricalreinforcement member (for a third layer) 37c having a diameter of 260mm×height of 400 mm, a bottomed cylindrical reinforcement member (for afourth layer) 37d having a diameter of 290 mm×height of 450 mm and abottomed cylindrical reinforcement member (for a fifth layer) having adiameter of 310 mm and height of 500 mm, each made of Cu material havinga thickness of 1 mm were prepared.

Then, the sheet-shaped cold accumulating material 36b containing Er₃ Nimagnetic particles was bonded to the external surface of the respectivereinforcement members 37c, 37d, 37e integrally, and consequently, theheat shielding members 39c, 39d, 39e as shown in FIG. 5 were prepared.

The heat shielding members 39a-39e for the first layer-fifth layerprepared as described above were disposed coaxially in the vacuumcontainer 33 as a heat shielding member of the low temperature coldreserving unit 30 shown in FIG. 10 so as to produce a low temperaturecold accumulating unit for cooling a superconductive magnet 31.Meanwhile, as the heat shielding members for the sixth layer-tenthlayer, the conventional heat shielding members 32 made of Cu materialhaving a thickness of 1 mm alone were disposed coaxially as shown inFIG. 11.

In the low temperature cold reserving unit 30 assembled in the abovemanner, totally ten layers of the heat shielding members 39a-39e, 32--32were cooled by the two-stage cooling type GM (Gifford MacMahon)refrigerator 10. After that, the heat switch 35 was turned off toseparate a thermal contact between the GM refrigerator 10 and heatshielding member. In this condition, the surface temperature of the heatshielding member 39a was measured. As a result, the temperature whicharrived at 4.0 K by the cooling operation of the GM refrigerator 10still remained at 5.0 K 100 hours after the GM refrigerator 10 wasseparated and thus, an excellent heat shielding performance could beconfirmed.

EXAMPLE 8

HoCu₂ alloy ingot was prepared by high frequency melting method, thisingot was mechanically cut and subjected to grinding work so as toprepare a plurality of chip like magnetic cold accumulating materialseach 20 mm vertically, 20 mm horizontally and 3 mm thick.

On the other hand, a bottomed cylindrical reinforcement member havingthe same size as the reinforcement members 37a, 37b for the first layerand second layer used in the Example 7 was prepared. Among theaforementioned chip-shaped magnetic cold accumulating materials, thechip-shaped magnetic cold accumulating material to be joined to a sideof the aforementioned reinforcement member was subjected to finishingwork to produce a curved surface coinciding with a curvature of the sideof the reinforcement member. On the other hand, the chip-shaped magneticcold accumulating material to be joined to a bottom of the reinforcementmember was kept flat.

The chip-shaped magnetic cold accumulating material finished to thecurved surface was bonded to a side of the aforementioned reinforcementmember (made of Cu) through ethyl-2-cyanoacrylate adhesive instantaneousadhesive agent (Aron Alfa: made by Toa Gousei Chemical Industry Co.,Ltd)and the flat chip-shaped magnetic cold accumulating material was bondedto the bottom of each reinforcement member in the same manner. Becausethe square-shaped chip-like magnetic cold accumulating member wasincapable of a circular bottom of each reinforcement member, a chipcoming out of the periphery of the bottom was finished to a shape ofthat periphery. As a result, a heat shielding member (for the first andsecond layers) in which the chip-shaped magnetic cold accumulatingmaterial was bonded to the reinforcement member integrally was prepared.

On the other hand, by bonding the chip-shaped magnetic cold accumulatingmaterial made of Er₃ Ni processed in the same manner to the side andbottom of the reinforcement members (for the third-fifth layers) made ofCu prepared in the Example 7, the heat shielding materials for the thirdlayer-fifth layer were prepared.

By disposing the heat shielding members for the first layer-fifth layerprepared in the above manner coaxially in the vacuum container 33 asheat shielding member for the low temperature cold reserving unit 30shown in FIG. 10, a low temperature cold reserving unit for cooling thesuperconductive magnet 31 was assembled. As the heat shielding membersfor the sixth layer-tenth layer, the conventional heat shielding members32 made of only Cu material having a thickness of 1 mm were disposedcoaxially as shown in FIG. 11.

In the low temperature cold reserving unit 30 assembled as describedabove, totally ten layers of the heat shielding members were cooled bythe two-stage cooling type GM (Gifford MacMahon) refrigerator 10 andafter that, the heat switch 35 was turned off so as to separate athermal contact between the GM refrigerator 10 and heat shieldingmembers. Then, in this condition, the surface temperature of the heatshielding material was measured. As a result, by the cooling operationof the GM refrigerator 10, the temperature which arrived at 4.0 K stillremained at 6.7 K even 100 hours after the GM refrigerator 10 wasseparated, and an excellent heat shielding characteristic was confirmed.

EXAMPLE 9

Nd metallic lump was hot-rolled in inert gas environment so as toprepare a sheet-shaped magnetic cold accumulating material having athickness of 3 mm. On the other hand, the aforementioned sheet-shapedmagnetic cold accumulating material was bonded to the external surfaceof the Cu made reinforcement members of the first layer-fifth layerprepared in the Example 7 using epoxy adhesive agent (Sumikadain: madeby Sumitomo Kagaku Kogyo Kabushiki Kaisha). To improve heat transferbetween the respective Nd made magnetic cold accumulating material andCu made reinforcement member, both the members were screwed togetherwith fixing screws 38 in the same method as shown in FIG. 5.

The heat shielding members for the first layer-fifth layer prepared inthe above manner were disposed coaxially in the vacuum container 33 asheat shielding member of the low temperature cold reserving unit 30shown in FIG. 10 to assemble a low temperature cold reserving unit forcooling the superconductive magnet 31. As the heat shielding members ofthe sixth layer-tenth layer, the conventional heat shielding members 32made of only Cu material having a thickness of 1 mm as shown in FIG. 11were disposed coaxially.

In the low temperature cold reserving unit 30 assembled as describedabove, totally ten layers of the heat shielding members were cooled bythe two-stage cooling type GM (Gifford MacMahon) refrigerator 10 andafter that, the heat switch 35 was turned off so as to separate athermal contact between the GM refrigerator 10 and heat shieldingmembers. Then, in this condition, the surface temperature of the heatshielding member was measured. As a result, by the cooling operation ofthe GM refrigerator 10, the temperature which arrived at 4.0 K stillremained at 8.2 K even 100 hours after the GM refrigerator 10 wasseparated and an excellent heat shielding characteristic was confirmed.

COMPARATIVE EXAMPLE 5

A low temperature cold reserving unit of the Comparative Example 5 wasassembled by the same structure as the Example 7 except that the all theheat shielding members of the first layer-tenth layer were configured ofthe conventional heat shielding members made of only Cu material havinga thickness of 1 mm as shown in FIG. 11.

In the low temperature cold reserving unit 30 assembled as describedabove, totally ten layers of the heat shielding members were cooled bythe two-stage cooling type GM (Gifford MacMahon) refrigerator 10 and theheat switch 35 was turned off so as to separate a thermal contactbetween the GM refrigerator 10 and heat shielding member. In thiscondition, the surface temperature of the heat shielding member wasmeasured. As a result, by the cooling operation of the GM refrigerator10, the temperature which arrived at 4.0 K rose to 22 K rapidly 100hours after the GM refrigerator 10 was separated and it was confirmedthat the heat shielding effect was low.

The above described examples show heat shielding members in which asheet like or chip like magnetic cold accumulating material was bondedto the external surface of the reinforcement member made of Cuintegrally. The magnetic cold accumulating material is capable ofexerting the same heat shielding characteristic even if it is bonded toeither outside or inside of the reinforcement member.

By bonding a magnetic cold accumulating material 40, 40 on both sides ofthe reinforcement member 37 as shown in FIG. 6, a heat shielding member41 excellent in heat shielding characteristic providing a high coldaccumulating effect was obtained. Further, it is permissible to form aheat shielding member 44 having double structure of the reinforcementmembers 42 and having cold accumulating powder 43 contained therebetweenas shown in FIG. 7. Further, it is also permissible to form a heatshielding material 46 in which the cold accumulating powder 43 wasloaded in a pipe-shaped reinforcement material 45 as shown in FIG. 8. Inthis case, the binder may be mixed with the cold accumulating powder 43as required.

The heat shielding members according to the above described Examples canbe processed to an arbitrary shape and are capable of maintaining anobject to be cooled under low temperatures for a long time, so that thetemperature stability of an apparatus operating under extremely lowtemperatures such as a superconductive wire, superconductive device,infrared ray sensor and the like can be improved largely.

INDUSTRIAL APPLICABILITY

As described above, the cold accumulating material for extremely lowtemperature cold according to the present invention is formed by fillingthe pores of the porous carrier excellent in processing characteristichaving a low gas-passing resistance with magnetic particles so that themagnetic particles having a high brittleness are supported by the porouscarrier easy to deform. Therefore, this material is easy to process to ashape having a small pressure loss such that the magnetic particles arenot cracked or damaged. Therefore, if the material is used as a coldaccumulating material for a refrigerator operated in high speed cyclessuch as the starling refrigerator and pulse tube refrigerator, anoperation having a small pressure loss and a high heat exchangeefficiency is enabled so that a refrigerator having a high refrigeratingcapacity can be realized.

Further, the heat shielding member according to the present inventioncan be easily processed to an any shape and is capable of maintaining anobject to be cooled under low temperatures for a long time.

We claim:
 1. A cold accumulating material for extremely low temperaturecold wherein pores of a porous carrier thereof are filled with magneticparticles containing rare earth element.
 2. A cold accumulating materialfor extremely low temperature cold according to claim 1, wherein saidporous carrier is made of a sheet-shaped porous metal or a meshed metal.3. A cold accumulating material for extremely low temperature coldaccording to claim 1, wherein a porosity of said porous carrier is 90%or more.
 4. A cold accumulating material for extremely low temperaturecold according to claim 1, wherein said porous carrier is a foamedmetal.
 5. A cold accumulating material for extremely low temperaturecold according to claim l, wherein said porous carrier is formed in aform of a sheet and a plurality of convex portions are formed on atleast one surface of said porous carrier.
 6. A cold accumulatingmaterial for extremely low temperature cold wherein magnetic particlescontaining a rare earth element are mixed with binder, solvent,dispersant and plasticizer so as to prepare a uniform slurry and theuniform slurry is molded to form a sheet-shaped molded body so that saidmagnetic particles are bonded to each other.
 7. A cold accumulatingmaterial for extremely low temperature cold according to claim 6,wherein a plurality of gas-passing holes are disposed in saidsheet-shaped molded body comprising the magnetic particles.
 8. Arefrigerator including a cold accumulating unit filled with the coldaccumulating material for extremely low temperature cold according toany one of claim 1 to
 7. 9. A refrigerator according to claim 8, whereinsaid cold accumulating material for extremely low temperature cold isloaded in said cold accumulating unit such that said cold accumulatingmaterial is wound in a shape of a roll.
 10. A refrigerator according toclaim 8, wherein said cold accumulating material for extremely lowtemperature cold is formed of a plurality of plate-shaped coldaccumulating elements each having a plurality of gas-passing holes andsaid plurality of the cold accumulating elements are stacked in multiplelayers in an axial direction of said cold accumulating unit.
 11. A heatshielding member for extremely low temperature wherein a coldaccumulating material for extremely low temperature cold according toany one of claim 1 to 6 is integrally bonded to a reinforcement membermade of a different material from said cold accumulating material forextremely low temperature cold.
 12. A heat shielding member forextremely low temperature according to claim 11, wherein saidreinforcement member is made of metallic material of at least one kindselected from Cu, Al, Fe, Ni and an alloy constituted mainly of saidmetallic material.
 13. A heat shielding member for extremely lowtemperature according to claim 11, wherein said cold accumulatingmaterial for extremely low temperature cold is a sheet-shaped coldaccumulating material formed by filling pores of a porous carrier withmagnetic particles together with a binder.
 14. A heat shielding memberfor extremely low temperature according to claim 13, wherein said coldaccumulating material for extremely low temperature cold and saidreinforcement member are bonded to each other with said binder.
 15. Acold reservation unit comprising a refrigerator cooling asuperconductive magnet, and a plurality of heat shielding members forextremely low temperature according to claim 11 disposed coaxiallyaround said superconductive magnet, wherein each of said heat shieldingmembers has a cyclindrical shape or curved shape.